1. Field of the Invention
The invention relates to a method for simulating the driving behavior of vehicles on a test stand in which the engine of the vehicle is coupled on the test stand to an electronically controllable braking apparatus and a simulation model calculates simulation values of variables which are representative of the driving state of the vehicle in that the reaction of the vehicle to the behavior of the engine and the values of the variables as determined immediately prior thereto are calculated, with at least the vehicle speed and the slip occurring in the driving wheels being calculated as variables.
2. The Prior Art
The behavior of the motor vehicles can be simulated on test stands. Usually, an internal combustion engine which is identical to the internal combustion engine disposed in the vehicle is coupled with an electric braking apparatus on a dynamic test stand. On the basis of various simulation models which are described below, a braking torque is determined which is set on the electric brake and burdens the internal combustion engine accordingly. From a systematic viewpoint one can distinguish between the following simulation models:                The drive train model MODD which reflects the masses, elasticities and dampings of the drive train as well as the speed increasing ratio of the change speed gear.        In a vehicle model MODV substantially the air resistance and the rolling resistance of the vehicle depending on the vehicle speed and the slip occurring in the drive wheels is taken into account. Moreover, the mass of the vehicle is reflected.        In a wheel model MODW the occurring slip is determined depending on the vehicle speed and the moment applied to the driving wheels.        
The partial models as described above can naturally be parts of an integrated overall model. In the description below the individual partial models will be discussed separately for the purpose of better clarity.
It is known to operate a test stand according to a simplified model where the slip is neglected. It is understood that effects which depend substantially on the slip cannot be reflected by such a model. In order to minimize errors resulting therefrom, a slight change of the air resistance or the rolling resistance of the vehicle is usually made in the model in order to ensure that the behaviour of the vehicle simulated on the test stand corresponds to the highest possible extent to the real vehicle.
According to an extended model according to the state of the art, the calculated slip for determining the vehicle speed is used in a calculatory correct manner. As a result, in stationary or close to stationary vehicle states it is possible to achieve a very favorable representation of the vehicle on the test stand. In connection with slip control systems the disadvantages as explained above occur, which are that the oscillations caused by the slip control cannot be reflected adequately.
The engine control of the vehicle can be provided in many different ways. In a first variant (a drive-determined system) the engine control is performed primarily by the driver, i.e. the driver influences the position of the throttle valve or any other relevant parameter in a substantially direct fashion. The motor vehicle system thus represents an open control system, i.e. the vehicle behavior acts back on the engine merely via the speed on the crankshaft. No other reactions are provided.
In another operating mode the engine control is not only influenced by the driver's intentions as expressed in the gas pedal position, but also by variables which depend on the behavior of the vehicle per se. Examples for such engine controls are cruise controls and slip control systems. In a slip control system the slip present in the driving wheels is determined from the speed difference of the driven wheels from the non-driven wheels. Interventions are made in the engine control depending on the slip. This can occur by a change of the throttle valve position, a change of the injected fuel quantity, the injection time or temporary cylinder cut-out. In this way it is possible in racing sports to keep the slip within an optimal region which ensures a maximum propulsive thrust or an optimal lateral guiding force of the driving wheels in curves.
The characteristic aspect in using slip control programs is that oscillations occur in the drive train when the slip control responds, which oscillations are caused by the control algorithms of the slip control program. These oscillations have a frequency of 20 Hertz for example.
It has now been seen that even in highly dynamic test stands and when using correct models for the drive train, the vehicle and the wheels, it is not possible to represent these oscillations in a manner which corresponds to the behavior of the real vehicle. The reason for this is the fact that the moment of inertia of the electric braking apparatus on the test stand is substantially higher than the moment of inertia of the of the driving wheels and the drive train. In a test stand for racing engines the moment of inertia of the electric braking apparatus is typically within a magnitude which is a multiple of the moment of inertia of a driving wheel. Due to this higher inertia on the test stand, the oscillations which are caused by slip control cannot be represented accordingly and it is thus not possible to obtain a realistic picture of the behavior of the vehicle in this operating state.
Even in the simulation of vehicles which are produced in series and in which a slip control generally only intervenes in exceptional driving situations, effects occur frequently which cannot be represented or can only be represented inadequately in a simulation performed in a conventional manner such as impacts in the drive train and the like.
On the basis of the mechanical loads on the test stand it is also not possible without special measures to reduce the moment of inertia of the electric brake to such an extent that a correspondence with the real vehicle can be produced.